Many subsurface natural resources, such as oil bearing formations, can no longer be exploited by drilling wells having vertical boreholes from the surface. Extended reach wells, such as wells drilled from platforms or "islands" and having long non-vertical or inclined portions, are now common. The inclined portion is typically located below an initial (top) nearly vertical portion. The deviated portion may have an inclined angle from the vertical that may approach 90 degrees (i.e., nearly horizontal). The result is a well bottom laterally offset from the top by a significant distance.
Current technology can produce boreholes at almost any incline angle, but current drilling (including completion) methods have experienced problems in long, highly deviated well bores. For example, running casing into some highly deviated holes can result in significantly increased drag forces (i.e., a high drag borehole). This can result in a stuck casing pipe string before reaching the desired setting depth of the casing. If sufficient additional force (up or down) cannot be applied to free the stuck casing, the result may be the effective loss of the well. Even if a stuck string is avoided or freed, the forces needed to overcome high drag may cause serious damage to the pipe.
In order to avoid unwanted drilling problems, indicators of these problems are predicted and/or monitored. For example, the lifting force (i.e., supported or indicator weight) required to support the weight of a casing string is not equal to the actual weight of the casing string in part because of drag forces in the borehole which (if large enough) can cause a stuck casing. The excess of actual weight compared to indicator weight (force required to support the casing) during running into a wellbore is an indicator of drag and the potential for a stuck casing. Other widely used drag related indicators include drilling speed and torque applied during rotary drilling. Other problems, some of which may be accentuated by highly deviated wells, include lost circulation, structural failure of the drill string, misdirection, cement failure, vapor/low density material segregation and pockets, and hole cleaning. Still other indicators for these and other problems during drilling include: mud return rate, density and temperature; mud pump pressure; well surveys; applied torque; cutting speed; string weight; and quantity of cuttings recovered.
Options to mitigate the risk of these problems are available, if indicated to be required. For example, high drag mitigation methods can either 1) add downward force or 2) reduce the coefficient of friction, e.g., by lubrication or conditioning of the borehole.
However, these mitigation options are generally costly and of limited effectiveness. For example, only a limited added downward force can be exerted on the pipe string. Excessive downward force beyond safe limits tends to buckle the string, adding still further drag forces (if laterally supported in a highly deviated well bore) or causing structural failure (if laterally unsupported). In addition, drilling with large added downward forces may be impractical or rig/tubular pick up weight limits may be exceeded.
Similar limits affect current coefficient of friction reducing (i.e., lubricating, hole conditioning, or drag reducing) methods. As longer lubricated pipe strings are run into an extended reach well, even a lubricated string will eventually generate unacceptable drag forces because friction is only reduced, not eliminated. The geometry and borehole wall (i.e., interface surface) conditions of some holes may also create increased resistance (high drag) conditions even with lubricated strings in shorter inclined vertical hole portions.
Many drilling variables and other factors which may significantly affect the drilling process can change drastically during the drilling or running of tubulars (i.e., casing running or tripping) and related operations. For example, drag forces at any instant of time may be calculated from actual torque and supported weight data indicators, but both can change quickly. These indicators are dependent upon many drilling (including formation) variables or other factors. Although some variables are relatively constant and known (such as pipe section stiffness), others (such as friction factor) can change quickly and are uncertain. These uncertain and changeable variables and factors also include borehole cross-sectional geometry, drill string ledge contacts, key seat effects, cutting bed properties, differential pressure effects, slant angle, contact surface, hydrodynamic viscous drag, bit balling, mud solids content and dog leg severity conditions.
Basic predictive analysis methods are used to plan a drilling program which is acceptable, i.e., likely to be successful. Expected drilling variable data are used in a model to predict a single likely value of each indicator of an unwanted condition. If some of the predicted values (during drilling) of an indicator (such as indicator weight) fall outside an acceptable or "normal" threshold, corrective or mitigation measures are planned and/or implemented. If mitigation measure is planned/implemented, a second prediction of the single likely value of each indicator using mitigated drilling variable values may be made to verify that the predicted value of each indicator is now acceptable.
Basic monitoring type techniques obtain drilling indicator (as well as some drilling variable) data during drilling (and completion) operations and compare these actual or real time monitored values to expected or threshold values. If a threshold value is exceeded or actual data are outside a "normal" range, the operator is warned of the danger so that other drilling method (mitigation measures) can be employed. One can also combine prediction and monitoring methods on an incremental basis, e.g., a different method for each zone or formation of interest.
A statistical approach, as described in U.S. Pat. No. 4,791,998, is also known. It first requires grouping of drilling data (i.e., indicator data and other factors) from a first set of similar wells that displayed an unwanted condition, e.g., a stuck pipe string. A second set of drilling data from another statistically significant group of similar wells that did not display the unwanted condition is also required. The method statistically analyzes drilling variables for a new well of interest with respect to these two prior data sets and predicts which group the well of interest is expected to fall into. If an unwanted condition is expected, mitigation measures are implemented to change the drilling variables towards values approaching the second set.
These methods have led to three types of drilling approaches, all three of which may result in excessive cost because of the inability to economically handle the inherent uncertain and variable factors such as downhole conditions. The first type, or excessively conservative approach, employs unnecessary mitigation measures to avoid problems which probably would not have occurred (i.e., the conservative threshold values for indicators signal potential problems along with false alarms and mitigation measures are frequently employed). Unless a significant risk of a problem occurring exists, employing a mitigation measure is not cost effective.
Unnecessary delay/failure to employ an effective or correct mitigation measure when needed is the sometime catastrophic result of an excessively risky second approach which ignores a significant chance of the unwanted condition (i.e., the threshold indicator value signals problems only after high risk of the problem exists, but with few false alarms and mitigation measures are infrequently employed). If a significant risk of a problem occurring exists, mitigation measures may be needed immediately, not after the problem surfaces. The most cost effective mitigation measure at an early step of the drilling plan may not be effective later.
The last of the three, or a statistical risk analysis approach balances the cost and risk of the two aforementioned approaches, but requires costly sets of well failure and well success data to supply a statistical model. However, even this sophisticated probabilistic technique has not been able to reliably avoid the risks of failure or unnecessary mitigation measures in all cases even when sufficient data is available. Sufficient statistical data may also not be available for exploration wells.
A simplified analysis method is needed to allow the drilling of extended reach wells, without unnecessarily implementing costly problem mitigation measures or accepting unnecessary risk. The method should also not require extensive data.